162 



COMPARATIVE ANATOMY. 



position of the anus, while the Sipunculidae seem to differ more 

 from them. But this position of the anus is really only a further 



development of that dorsal posi- 

 tion which it has in many of the 

 Vermes, and does not in any 

 way affect the homology of their 

 enteron with that of the other 

 members of this group. 



1 QO 



IOji 



Fig. 73. En- 

 teric canal of 

 Sanguisuga. 

 o (Esophagus, 

 c Posterior pair 

 of ca^ca. a 

 Anus. 



Fig. 74. Enteric canal 

 of Aphrodite, o An- 

 terior portion. & Middle 

 (muscular) portion of 

 the fore-gut. c Branched 

 caacal appendages of 



The metamerism of the body 

 in the Annulata affects the en- 

 teric tube ; but there are various 

 other differentiations in it, which 

 are due to adaptations to special 

 modes of life. Here again the 

 enteric canal begins as a csecal 

 invagination. The aproctous 

 condition, which persists in most 

 of the Platyhelminthes, is passed 

 through by these forms at an 

 early stage in development. The 

 entrance to the fore-gut is most 



the mid-gut. a Anus. 



-a 



variously differentiated in the 

 Hirudinea. In some the pro- 

 tractile oesophagus is greatly 

 complicated, in others its en- 

 trance is armed with chitiuous ridges, which are the first signs of 

 jaws. But in most the mid-gut is beset by pouch-like diverticula 

 (Fig. 73), which are branched in Clepsine; the last two of these 



diverticula sometimes form longer caecal tubes (c) 

 on the narrow hind-gut, which extends to the 

 end of the body (Clepsine, Hsemopis). These are 

 the only caeca of the gut of Aulacostomum. In 

 others the caeca are merely indicated by constric- 

 tions. In all cases these arrangements correspond 

 to the metamerism expressed also in the nerve- 

 chord. 



In nearly all Annelids the fore-gut is separated 

 into several, often very different, portions. A 

 median portion is distinguishable by its more 

 powerful muscular investment, and is separated 

 from the mid-gut by a tract of varying length. 

 Among the Scoleina, this portion, which is known 

 as the " muscular stomach," is very greatly de- 

 veloped (Lumbricus). It forms the end of the fore-gut. It is placed 

 more towards the middle of the latter in most Chaetopoda, and is 

 frequently provided with denticles, which work on one another like 



Fig. 75. Maxillary 



apparatus of a Euni- 



cea (Lysidico). a-e 



Pairs of jaws (after 



Milne-Edwards) . 



